1. Field of the Invention
The present invention generally relates to an image forming apparatus which includes a fixing apparatus that fixes an unfixed toner image onto a recording material.
2. Description of the Related Art
Conventionally, a heat fixing apparatus which includes a fixing member, such as a fixing roller, that is heated by a heater and a pressure member, such as a pressure roller, that is pressed against the fixing member has been known and widely adopted into an image forming apparatus. The heat fixing apparatus nips and conveys a recording medium such as a paper or the like (hereinafter referred to as a sheet) that carries an unfixed toner image at a nip portion formed between the fixing member and the pressure member, and fixes a toner image onto the sheet by heat and pressure.
Further, a fixing apparatus which utilizes an endless belt as a fixing member has been known. According to the fixing apparatus, it becomes possible to shorten a warming-up time and to achieve energy-saving, since the thermal capacity of the endless belt is relatively small.
FIG. 1 shows a side view of an example of a conventional heat fixing apparatus. A sheet S on which a toner image 105 is transferred is fed into a fixing apparatus 110. The toner image which is transferred onto the sheet S is fixed by heat and pressure at a nip portion N where a fixing roller 111 and a pressure roller 112 nip the sheet S therebetween.
A problem in that the sheet S is wrapped tightly around the fixing roller 111 and is not conveyed properly after passing through the nip portion N may occur. Thus, a technique that strips the sheet S from the fixing roller 111 by pressing a scraper 108 or the like onto the fixing roller 111 has been widely known. However, a problem in that the surface of the fixing roller 111 is attrited by being pressed by the roller 111 and thereby a printed image is defected, and a problem in that contamination such as toner is accumulated onto the scraper 108 and thereby the sheet S is spotted have been known.
A technique which solves the problems described above has been proposed by for example Japanese Patent Laid-Open Publication No. H03-081791 (patent document 1). According to the technique, the sheet is stripped from the fixing roller by blasting compressed air to the fore-end of the sheet. Patent document 1 discloses a sheet stripping apparatus which strips an electrophotographic print sheet from a fixing roller by blasting compressed air when the sheet is adhered to the fixing roller in a fixing procedure of the sheet. The sheet stripping apparatus includes an air pressure controller which controls air pressure of the compressed air according to the thickness of the sheet so that the sheet is stripped from the fixing roller.
Herein, blasting time of the compressed air which is required for stripping the sheet depends on feeding speed of the sheet, capacity of an air tank, an opening space of an exhaust nozzle of the compressed air, number of the exhaust nozzle or the like. For example, Japanese Patent Laid-Open Publication No. 2007-233228 (patent document 2) discloses that the blasting time of the compressed air is preferably set from 30 ms to 200 ms, and once is preferable number of times that the compressed air is blasted to the fore-end of the sheet. Patent document 2 discloses a fixing apparatus which includes an air blasting device that blasts air from downstream side of a nip portion to neighborhood of the nip portion. The air blasting device includes a first air blasting member which blasts air to the neighborhood of the nip portion and strips a recording medium from a heater, a second air blasting member which blasts air to a fixed recording medium and cools down the fixed recording medium, a selecting means for selecting the first air blasting member or the second air blasting member that blasts air, and an air guide member which is disposed in downstream side of the recording medium in the direction of a feeding path and guides the air blasted from the second air blasting member to a peripheral surface of a pressure welding member in order to cool down the pressure welding member.
Because of recent demands for a variety of electrophotographic devices, basis weight of the sheet ranges from 35 g/m2 to 199 g/m2. The blasting time or the air pressure that makes it possible to strip the sheet from the fixing roller or the like varies depending on the basis weight or thickness of the sheet.
For example, in a condition where feeding speed of the sheet is 700 mm/s, capacity of an air tank is 1300 ml, pressure of compressed air is 0.1 MPa, an opening space of an exhaust nozzle of the compressed air is 3 mm2, number of the exhaust nozzle is one, and printing speed is 150 sheets per minute, the blasting time that makes it possible to strip the sheet, which has basis weight of 75 g/m2, from the fixing roller or the like is empirically determined to 90 ms. The sheet which has basis weight of 75 g/m2 is most sought after.
However, in a case where amount of toner which is transferred onto the sheet is increased by variation in condition or environmental factors of the electrophotographic device, it becomes difficult to strip the sheet from the fixing roller, since adhesion of the sheet which is wrapped around the fixing roller becomes larger. Thus, a problem in that the electrophotographic device is shut down because of a sheet jam may occur.
This problem occurs in a case where a light weight sheet of which basis weight ranges from 35 g/m2 to 85 g/m2 is used, and notably occurs in a case where the basis weight of the sheet ranges from 35 g/m2 to 64 g/m2. Herein, as the basis weight of the sheet decreases, rigidity of the sheet decreases. As the amount of toner which is transferred onto the sheet increases, adhesion of the sheet to the fixing roller increases. Insufficient rigidity of the light weight sheet and increased adhesion of the sheet cause decrease of stability in stripping the sheet from the fixing roller. Thus, the problem is caused by the decrease of stability in stripping the sheet from the fixing roller. Even when the fore-end of the sheet is stripped from the fixing roller, a middle portion of the sheet may be adhered to the fixing roller and may cause middle portion jam because of the increased adhesion and the insufficient rigidity of the sheet.
For example, Japanese Patent Laid-Open Publication No. 2005-128333 (patent document 3) discloses a technique for controlling blast rate or blast pressure of compressed air according to image information. Patent document 3 discloses an image forming apparatus which includes a toner image forming means for forming unfixed toner image, a fixing means which includes a heat roller and a pressure roller, and a strip guide plate of which a side is disposed adjacent to the surface of the heat roller, and gas blasting device which blasts pulsed compressed gas to a portion between the surface of the heat roller and the surface of the strip guide plate. The image forming apparatus further includes a strip means for stripping a recording medium from the heat roller and a blast controller for controlling an operating condition of the gas blasting device according to the image information and/or information about the recording medium.
The technique disclosed in patent document 3 detects a planar dimension of an area on which the toner is transferred, and strips the sheet by controlling the blast rate or the blast pressure so that the blast rate or the blast pressure meets the planar dimension of the area. It becomes possible to utilize the compressed gas efficiently by increasing the blast rate or the blast pressure in a case where the planar dimension of the area is large, or by decreasing the blast rate or the blast pressure in a case where the planar dimension of the area is small.
However, an operating principle of an image forming of the electrophotographic device is very complicated, and includes lots of variation factors. Thus, the amount of toner which is transferred onto the sheet is affected by the variation factors. Therefore, it is difficult to estimate increase and decrease amount of the toner transferred onto the sheet accurately based only on the image information. The sheet jam may occur in a case where adhesion of the sheet is increased by increased amount of toner transferred onto the sheet, even though the planar dimension of the area is relatively small. On the contrary, the sheet may be stripped stably and easily in a case where adhesion of the sheet is decreased by decreased amount of toner transferred onto the sheet, even though the planar dimension of the area is relatively large.
In order to solve the problems described above, it is necessary to set the blast rate or the blast pressure of the compressed gas all the time in a condition where the amount of the toner transferred onto the sheet becomes the maximum. However, this technique results in cost increase of the electrophotographic device and causes growth in size of a system including the electrophotographic device, since this technique requires large quantity of the compressed gas in order to increase capability of a compressed gas generating device. Further, this technique requires large amount of power for generating large quantity of the compressed gas, and results in increase of environmental load. Furthermore, this technique results in increase of heater power in order to counter cool down of the fixing roller performed by the increased compressed gas.